example of calculation of rectangular steel tank

Surface loading rates in clarifiers are often obtained by dividing the daily or hourly flow by the gross surface area of the tank. the theoretical time of travel of water in the tank, varies from a minimum of 1½ hours to an average of 4 hours, but is more usually about 2½–3 hours. Detention times for particles of low relative density, i.e. Circular tanks typically have a side water depth of 4.5 m. The depth of the rectangular tanks should be adequate for sludge deposits and storage a minimum of 3 m is recommended for scraped tanks and 5 m recommended for manually cleaned tanks ( Section 8.19). Alternative arrangements include ‘finger’ launders of length <20% of the tank length (rectangular tanks) or radial launders (circular tanks) spaced at less than 8 m on the circumference feeding a collector launder. These are placed about 0.5 m from the end wall (rectangular tanks) or peripheral wall of circular tanks. To achieve the outlet flow, double sided launders are usually used. They should be about 75 mm below the water surface or more (0.1–0.2 m) below the water surface where freezing is a problem, depending on the thickness of the ice layer. Orifices should be sized to give a flow velocity of about 0.6–0.7 m/s with an orifice diameter greater than 30 mm and headloss 35–40 mm. Submerged orifice outlets are useful in minimizing the passage of floating material to filters or where freezing is likely. Alternatively, 90° V-notches with a water depth of about 75 mm and spacing less than 0.6 m may be used. The flow over the outlet weir should be maintained below 50 m 3/h per m of weir length and should preferably be about a quarter of this value. The mean horizontal flow velocity should be in the range 0.25–1.0 m/min.

Where γ is kinematic viscosity (m 2/s) and g=9.81 m/s 2.Ĭhemically assisted sedimentation tanks for the removal of organic matter and light flocculated particles, have length to width ratios greater than or equal to 4:1 and surface loading rates between 0.75 and 1.75 m 3/h.m 2, which, with a coagulant aid, may increase to 2.5 m 3/h.m 2. Re = Q b + 2 y 1 γ and F = S 2 g L 2 ( b + 2 y ) 1.3 × 10 7 b y 3 Whilst these measures help, they may have little effect when the temperature of the incoming water differs from that in the tank and so creates turbulence which must subside before the particles can resume settling under quiescent conditions. These design features for rectangular tanks include multiple inlets at about 1.5 m centres sized to give an inlet velocity of about 0.5 m/s and perforated baffles, with orifice diameters of 100–200 mm to give a headloss of less than 10 mm to minimize floc shear, at the inlet and outlet ends across the whole cross section of the tank to create a more uniform flow pattern through the tank. The safety factor can be limited to between 1.3 and 1.5 by careful design of the inlet and outlet arrangements. However, theoretical settling rates cannot be directly translated to tank design because of short-circuiting in the tank. With coagulant assisted sedimentation, the actual efficiency is generally much closer to the results obtained by laboratory jar tests. Without the aid of a coagulant, a rectangular or circular tank only works at a surface loading up to about one third the rate shown by jar tests (33% efficiency). Ratnayaka BSc, DIC, MSc, FIChemE, FCIWEM, in Twort's Water Supply (Seventh Edition), 2017 8.15 Design Criteria Basis: Inside Measure, 1 cubic foot = 7.481 gallons (U.S.)

Example of calculation of rectangular steel tank